A mouse is a device for inputting data into a computer. A mouse rolls on a mouse pad and, through mechanical or other means, sends positional and data selection information to the computer's processing unit. The term puck is also used to refer to a mouse-like input device, especially in the context of a graphic system. The puck often operates in concert with a tablet. As used in this document, the term "mouse" implies both mouse and puck input devices.
The mouse may be attached to the computer via a cable or it may use various cordless technologies to communicate with the computer. On the top side of the mouse are one or more command buttons. The number and exact arrangement of these buttons varies from vendor to vendor.
Although mice may differ in their exact style and appearance, their function and method of use is very similar, regardless of exact model. To communicate with the computer, the computer user must move the mouse and press selection buttons. Pressing and releasing the selection buttons is called clicking. This repetitive moving and clicking, as well as other activities associated with using the mouse, can cause tendinitis and other types of hand, wrist, and arm pain, referred to in general as Repetitive Stress Injuries (RSIs) or Cumulative Trauma Disorders (CTDs). Once the pain is present, a vicious cycle of use-pain-stop use/use-pain-stop use often begins and computer use becomes difficult.
Four factors contribute to cumulative trauma disorders:
Highly repetitive motions PA1 Mechanical stresses PA1 Fast forceful exertions PA1 Awkward postures
One of the more damaging types of repetitive motions is the continual flexion and extension of the middle finger, as might be caused by clicking the mouse button. Constant flexion and extension of the middle finger causes activation of the extensor carpi radialus brevis muscle and may cause subsequent impingement and irritation of the radial nerve.
Mechanical stresses are produced when soft tissues are squeezed between bone and external objects, such as the edge of a computer mouse. The magnitude of these stresses is related to the contact force and the area of contact. The greater the force and the smaller the contact area, the greater the stress. Tools that rub the sides of the fingers create mechanical stress that may cause digital nerve compression. Tools supported over the base of the palm may stress the median nerve and result in carpal tunnel syndrome.
Force is produced by contractile proteins in muscles and is transmitted through myofacial sheaths, tendons, bones, and ligaments. It results in elastic and viscous deformation of tissues. Risk of CTDs increases with the force of exertion. To cause problems, the exertion need be only as forceful as gripping a mouse. Muscles that stay tensed and still for a long time can become even more tired than muscles in motion. Gripping a mouse for extended periods of time as is common in computer work creates a high level of force of exertion.
Incorrect finger, hand, and wrist posture both increases force of exertion and is in and of itself a risk factor for cumulative trauma disorders. Movements of the wrist such as flexion, extension, and radial or ulnar deviation cause the tendons to be displaced past and against adjacent anatomic surfaces. Flexion and extension of the wrist are associated with tenosynovitis of the flexor and extensor tendons in the wrist and with carpal tunnel syndrome, one of the more common cumulative trauma disorders. Flexing the wrist causes the median nerve to be compressed between the finger flexor tendons and the flexor retinaculum, whereas extending the wrist cases the nerve to be stretched around the tendons. Nerves may also be stressed and compressed by exertion in certain postures, such as flexion of the fingers with the wrist flexed. Side to side deviations of the wrist are associated with tenosynovitis at the base of the thumb.
Incorrect mouse posture creates situations which stress the hand and put a user at risk of developing a CTD. The hand and wrist should be maintained in a neutral position, that is, neither flexed nor hyper-extended nor deviated side to side. This posture can be demonstrated by letting the arm and hand hang relaxed at the side of body.
CTDs are increasingly common; according to federal statistics, reports of cumulative trauma disorders increased 538 percent between 1981 and 1990. It is estimated that one of every four injuries on the job involve fingers, hands, or wrists and fully half of those are related to CTDs. In fact, CTDs are now a major cause of lost time in many hand-intensive industries.
Some current methods of dealing with this problem are to stop using the computer entirely (not always feasible), to use a different type of interface altogether (often costly and not always practical) or to irretrievably alter the mouse so that it suits the specific individual (not always desirable).